Response of a parasympathetic neuroeffector system to motor nerve stimulation

The pressure response of the feline urinary bladder to motor nerve stimulation was measured by a strain gauge connected to a urethral catheter. In lightly anesthetized cats the magnitude of the responses produced was a hyperbolic function of the stimulation frequency within the range of .5–20 impulses/sec. Blockade of the ChE system by the administration of 1 mg/kg of prostigmine with or without arrest of the bladder circulation failed to alter the characteristics of this response. Our findings suggest that these graded responses of bladder smooth muscle at different stimulation frequencies is dependent upon a mechanical summation of individual ‘twitch’ responses. The response magnitude is in a way proportional to the concentration of ACh. Contractions resulting from repetitive stimulation at rates of 10–15/sec., achieved optimal summation. At stimulation frequencies below 5/sec. the efficiency of mechanical summation in the bladder smooth muscle diminished. When the interval between stimuli was increased to 200 msec. (5/sec.) the summated response of the smooth muscle established a lower plateau. It is suggested that the smaller responses found at the lower frequencies may be related in part to the tendency of smooth muscle units to restore themselves to the resting state independently of high ACh concentrations.

Prejunctional alpha2-adrenoceptors and peroxide-induced potentiation of norepinephrine release from the bovine iris

Peroxides can enhance field-stimulated [3H]norepinephrine ([3H]NE) release in isolated irides from several mammalian species. In the present study, we investigated the role of prejunctional alpha2-adrenoceptors in peroxide-induced potentiation of sympathetic neurotransmission in bovine isolated irides. Isolated hemi-irides were incubated in a Krebs buffered-solution containing [3H]NE and prepared for studies of neurotransmitter release using the superfusion method. Alpha2-adrenoceptor agonists, oxymetazoline, UK-14304 and clonidine inhibited field-stimulated [3H]NE overflow without affecting basal tritium efflux. Pretreatment of tissues with H2O2 (300 microM) had no effect on inhibition of evoked [3H]NE release caused by the alpha2-adrenergic agonists. However, H2O2 (300 microM) caused significant (P < 0.01) leftward shifts of excitatory concentration-response curves to yohimbine (10 nM-1 microM). In contrast, yohimbine (1 microM) did not prevent the enhancement of evoked [3H]NE overflow induced by H2O2 (300 microM). In conclusion, excitatory effects of peroxides on sympathetic neurotransmission in bovine irides are not mediated by prejunctional alpha2-adrenoceptors.

Studies on the interactions between nerve fibres from para- and orthosympathetic ganglia and adreno-cortical and -medullary cells in joint culture

The interactions between nerve fibres from para- and orthosympathetic ganglia and adreno-cortical and -medullary cells were studied in joint cultures using explanted guinea-pig ciliary and sympathetic chain ganglia and enzyme-dispersed rat adrenal gland cells. Nerve fibres from both para- and orthosympathetic ganglia made only transitory contact with cortical cells, but consistently formed associations with isolated chromaffin cells which lasted for up to 10 days. Contacts between axons and chromaffin cells often showed particularly large varicosities and frequently withstood severe tests of durability from pulls of the fibre or the cell or both. By correlating phase contrast and catecholamine histochemistry (Falck-Hillarp method) it was shown that sympathetic fibres forming long-lasting contacts with chromaffin cells were adrenergic. The functional implications of the ability of autonomic nerves to distinguish between adreno-cortical and -medullary cells and the lack of specificity shown by the para- and orthosympathetic neurons during formation of long-lasting associations with chromaffin cells are discussed.

An investigation of spontaneous potentials recorded from the smooth-muscle cells of the guinea-pig seminal vesicle

1. A spontaneous discharge of small potentials from smooth muscle cells of the guinea-pig seminal vesicle has been described. The mean amplitude, frequency and duration of the potentials were 0.9 mV, 12.2/min, and 92 msec, respectively. Occasionally, large potentials of 5-7 mV were recorded.2. The frequency of spontaneous potentials was enhanced by about twofold in 15 mM potassium-Krebs solution. Removal of calcium from the perfusing solution, or adding excess magnesium (12 mM) suppressed the rate of these potentials. Cobalt (2.5 mM) did not affect them but blocked excitatory junction potentials evoked by the hypogastric nerve stimulation.3. The spontaneous potentials were unaffected by atropine. Their rate and amplitude were reversibly reduced in the presence of high concentrations of phentolamine (10(-4) but not by 10(-5) g/ml.). Guanethidine enhanced the frequency initially (5 min), but subsequently the control rate was not appreciably changed, even after 20 min of guanethidine perfusion.4. After post-ganglionic denervation or reserpine treatment, the frequency of discharge of spontaneous potentials was reduced, without any effect on amplitude.5. The properties of the spontaneous potentials recorded from the smooth muscle cells of the seminal vesicle are similar to those reported for the vas deferens. They appear to be due to an interaction between noradrenaline released from sympathetic nerve endings and the smooth-muscle cell membrane. Present experiments cannot completely rule out the possibility that another transmitter in addition to noradrenaline may be released at this neuroaffector junction.

Differences in the manner of autonomic innervation of the muscle layers of the guinea-pig ductus deferens

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